1 Shakespeare: I actually have some acting experience.
2 Shakespeare: I performed in an itinerant troupe of stage players in my youth.
3 Mercutio: I played a World War I Aussie Digger addicted to gambling in one of my high school's productions.
4 Shakespeare: Are you trying to one-up me?
4 Mercutio: No. Two-up you, mate!

Two-up is a gambling game in which someone throws two coins in the air at the same time, and people bet on whether the resulting tosses come up two heads (25% chance), two tails (25%), or "odds" (one head and one tail, 50% chance). There are various standard forms of betting, some of which accumulate results over multiple tosses depending on earlier results, a bit like the dice game of craps.

The game originated in England, and was taken to Australia in the 18th century, where it really took off as a pastime during the gold rushes. This led to it being a popular activity for the soldiers who enlisted to fight for Australia in World War I.[1] It was played so much by the Australian troops that it became entrenched[2] as a strong tradition. When the soldiers returned home after the war, two-up became a symbol of solidarity and comradeship between the veterans and those they left behind.

And so it came to be that on Anzac Day, the anniversary of Australia's disastrous assault on the beaches of Gallipoli on 25 April 1915, World War I veterans in Australia would gather to remember their fallen friends, and to engage in nostalgic games of two-up as a sort of tribute.

And this is why, as a gambling game, two-up is illegal in Australia on 364 days of the year, but it is legal on our most solemn day of remembrance, Anzac Day.

[1] WWI Australian soldiers were known as Diggers in a way similar to how U.S. soldiers were known as G.I.s during WWII.